Low temperature synthesis of metal sulfides

ABSTRACT

Finely divided metal sulfide powders of uniform size are produced at low temperatures by a method of adding solutions of organometallic compounds to an organic solvent saturated with H 2  S. The solvent is kept saturated with H 2  S by adding H 2  S at a rate greater than that for the organometallic compound.

BACKGROUND OF THE INVENTlON

1. Field of the Invention

This invention is related to metal sulfide powders useful as precursorsfor optical ceramics and catalysts. More particularly, this invention isrelated to a low temperature synthesis of metal sulfide powders byreaction between an organometallic compound and hydrogen sulfide.

2. Description of the Prior Art

Metal sulfide powders are useful as precursors for optical ceramics usedin sensor windows and domes on aircraft, satellites, and missiles.Precursors having high purity and small. uniform particles are necessaryto provide optical ceramics having excellent thermal, mechanical andoptical properties. Additionally, metal sulfide powders are useful ascatalysts because they contain small, uniform particles having a largesurface area.

Conventional methods of synthesizing metal sulfide powders require hightemperatures and inorganic starting materials. High temperatures promoteparticle growth which detracts from the optical and thermomechanicalproperties of the ceramic. Inorganic starting materials containundesirable impurities which can degrade the performance of the opticalceramic or catalyst.

Organometallic compounds are known to be useful in the preparation ofsome metal chalcogenides. For example, the gas phase reaction between anorganometallic compound such as diethyl zinc with hydrogen sulfide,hydrogen selenide or dimethyltellurium has.been reported. The reactionis run at temperatures of 750° C. or greater and has been used toprepare the sulfides, selenides and tellurides of zinc and cadmium.

The liquid phase reaction of diethyl zinc with hydrogen sulfide has beenreported. This room temperature reaction has been used to prepare zincsulfide containing ligands such as 2,2'-bipyridine, 1,10-phenanthroline,and pyridine. The procedure involves an H₂ S purge of an anhydrousethereal solution of diethyl zinc and an organic ligand. The procedurehas been adapted to produce zinc sulfide without the ligand. However,the particle size of the resulting zinc sulfide was not reported.

Optical ceramic precursors having small particles are desirable becausethey often can be processed under mild conditions into fine-grainedceramics. The resulting fine-grained ceramics are generally low indefects and have improved optical and mechanical properties. Moreover,if the grain size is substantially smaller than the wavelength of light,then light scattering between grains in non-cubic materials isminimized. As a result, a wider range of metal sulfides would beavailable as optical ceramics.

Metal sulfide powders used as optical ceramic and catalyst precursorsmust have few impurities. The purity of the final product is dependenton the purity of the starting material. Inorganic starting materialsused in conventional syntheses contain impurities which can degrade theperformance of the ceramic. Organometallic starting materials of veryhigh purity are easily obtained by distillation, sublimation orrecrystallization. However, the purity of the final product may beaffected by the presence of residual hydrocarbon in the form ofincompletely reacted organometallic material or entrained solvent.

SUMMARY OF THE INVENTION

Aocording to the present invention finely divided metal sulfide powdersof uniform size are produced by adding a solution of an organometalliccompound at a predetermined rate to an organic solvent saturated with H₂S. The reaction can be performed at temperatures in the range of -78° C.to 100° C. The reaction solution is kept saturated with H₂ S by addingH₂ S at a rate greater than that for the organometallic compound.

An object of this invention is to provide a low temperature synthesis ofmetal sulfides that may be used as precursors to optical ceramics orcatalysts.

Another object of this invention is a method of producing high puritymetal sulfide powders containing small, uniform particles having a largesurface area.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a synthesis of metal sulfide powders byreaction of an organometallic compound in solution with hydrogen sulfide(H₂ S) at temperatures from about -78° C. to about 100° C.Organometallio reagents are particularly suitable for the lowtemperature preparation of metal sulfide powders which are desired asprecursors to 8-12 micrometer infrared-transmitting ceramics.

The use of organometallic reagents in the low temperature synthesis ofceramic powders offers many advantages over conventional preparations.Organometallic reagents can be obtained in high purity and can be mixedat a molecular level while in solution. Bimetallic complexes may also beuseful for making ternary materials. Additionally, the low temperaturespossible with organometallic oompounds in solution are a factor inpromoting small and uniformly sized particles which are suitable forprocessing under mild conditions into fine-grained ceramics.

Metal sulfide powders have been prepared by reacting H₂ S withdimethylzinc, di(tertiary-butyl)zinc, triethylaluminum,trimethylaluminum, diethylmagnesium, andbis(trimethylsilylmethyl)cadmium. Other reagents besides metal alkylcompounds may be useful, such as metal alkoxides, thioalkoxides andhydrides.

The reaction of organometallic compounds in solution with hydrogensulfide produces a fine precipitate immediately upon mixing. The majorimpurity in the resulting metal sulfide powder is residual hydrocarbonwhich reduces the transparency of optical ceramic materials upondensification.

Residual hydrocarbon usually results from incomplete reaction of theorganometallic reagent or from residual organic solvent. The amount ofresidual hydrocarbon due to unreacted zinc-alkyl groups parallels theorder of stability of the zinc-alkyl compounds. For example,dimethylzinc is more stable than diethylzinc, which in turn, is morestable than di(t-butyl)zinc. Zinc sulfide powders made from dimethylzinc contain more unreacted metal alkyl reagent and, consequently, moreresidual hydrocarbon than powders made from diethylzinc ordi(t-butyl)zinc.

Additionally, certain solvents such as diethyl ether are difficult tocompletely separate from the metal sulfide powders and may alsosignificantly increase the amount of residual hydrocarbon. Solventswhich are easily separated from the metal sulfide powders includetoluene and heptane.

The method of adding reagents significantly affects the purity of theresulting metal sulfide powder. A H₂ S purge of an organometalliccompound in solution results in significantly higher levels of residualhydrocarbon than procedures involving the addition of an organometallicreagent in solution to an organic solvent that is saturated with H₂ S.For example, the H₂ S purge of a diethylzinc/heptane solution at 25° C.resulted in a residual zinc-ethyl group level of 30,000 ethane/molezinc) as measured by acid hydrolysis in which the liberated hydrocarbonis measured using gas chromatography. In contrast, adding adiethylzinc/toluene solution to toluene saturated with H₂ S at -20° C.resulted in a residual zinc-ethyl group level of about 10 ppm.

Temperature is also important in preparing small, uniform particles. lnthe range of temperatures used for the reaction, low temperatures resultin smaller particle sizes. One possible explanation may be a largernumber of nucleation sites at lower temperature, due to slower diffusionof reactants. Additionally, low temperatures increase the solubility ofH₂ S in the organic solvent. As a result, increased amounts of H₂ S inthe solvent are believed to cause more complete reaction of theorganometallic reagents leading to less unreacted organometalliccompounds. Zinc sulfide has been formed according to the presentinvention at temperatures ranging from -78° C. to 100° C. Temperaturesfrom -78° C. to about -100° C. may also be favorable for the reaction.Temperatures in the range from -78° C. to about -20° C. have been foundto give the best results. At temperatures near the boiling point of H₂S, a 1 Molar solution of H₂ S in the organic solvent is preferred.

The rate at which the organometallic compound in solution is added tothe H₂ S-saturated solvent affects the level of organic residue in themetal sulfide powder. A dilute solution of the organometallic compoundadded at a constant rate from about 0.01 moles/hour to about 1 mole hourhas been found to be suitable for a reaction on the 0.1 mole scale. Thepreferred rate of addition is about 0.1 mole/hour for a reaction on the0.1 mole scale. lt is also important to maintain H₂ S saturation of theorganic solvent during addition of the organometallic compound. This isaccomplished by bubbling H₂ S through the organic solvent at a rateslightly greater than the rate which the organometallic compound isadded.

The following example is given to illustrate but not limit theinvention:

EXAMPLE Synthesis of ZnS

Diethylzinc was distilled and stored in a helium-filled glovebox.Toluene and pentane were distilled from sodium and stored under argon.H₂ S was used as received.

Approximately 15.2 g (0.123 mole) of diethyl zinc was added to a dryflask under an argon atmosphere. About 90 ml of toluene was added to areaction vessel which was then cooled and maintained at a temperature of-20° C. to -25° C. The toluene in the reaction vessel was saturated withH₂ S by bubbling the gas through the solvent.

The diethylzinc was diluted to 1 mole per liter with 110 mL of tolueneand stirred briefly to ensure homogeneity. With rapid H₂ S flow throughthe H₂ S-saturated solvent and rapid stirring with a magnetic stirbar inthe reaction vessel, the diethylzinc solution was added to the toluenesolution via a 22 gauge cannula over a period of about 1 hour. After theaddition of the diethylzinc, the H₂ S flow was continued for 1 minute.The reaction vessel was allowed to warm to room temperature and theexcess H₂ S was allowed to escape.

The zinc sulfide powder was collected by filtration under an argonatmosphere and washed with two 30 mL portions of toluene and three 20 mLportions of pentane. The solid was pumped dry for 20 minutes and thentransferred to a glovebox. The solid was dried under a vacuum of lessthan 0.01 torr at 25° C. for one hour then heated and maintained at 100°C. for 24 hours.

Electron micrographs of the solid show spherical particles withdiameters ranging from 20 to 100 nanometers. Acid hydrolysis of thesolid shows a level of zinc-ethyl group impurity of approximately 10ppm.

Modifications and variations of the present invention are possible. ltshould be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically described.

What is claimed is:
 1. A low temperature method of producing metalsulfide powders with small, uniform size and a large surface area byreaction between an organometallic compound and hydrogen sulfidecomprising the steps of:saturating an organic solvent with H₂ S; addinga solution of an organometallic compound and organic solvent to said H₂S saturated solution; maintaining H₂ S saturation of said reactionmixture by adding H₂ S at a greater rate than the organometalliccompound; recovering a pure, finely divided powder; and drying saidpowder by heating while under a vacuum.
 2. The method of claim 1 whereinsaid organometallic compound is selected from the group consisting oftriethylaluminum, trimethylaluminum, diethylmagnesium, andbis(trimethlsilylmethyl) cadmium.
 3. The method of claim 1 wherein saidorganic solvent is selected from the group consisting of toluene andheptane.
 4. The method of claim 1 wherein said saturated solution of anorganic solvent and H₂ S is maintained at a temperature within the rangefrom about 100° C. to about -40° C.
 5. The method of claim 1 whereinsaid solution of an organic solvent and H₂ S is maintained at atemperature within the range from about -40° C. to about -100° C. and ata H₂ S concentration of about 1 mole per liter.
 6. The method of claim 1wherein said organometallic compound is added to said H₂ S saturatedsolution at a rate within the range from about 0.01 moles/hour to about1 mole/hour for a reaction on the 0.1 mole scale.
 7. The method of claim1 wherein said finely divided powder is recovered by filtration andwashing.
 8. The method of claim 1 wherein said powder is dried byheating under a vacuum of less than 0.01 torr to 100° C. for about 24hours.
 9. A low temperature method of producing zinc sulfide particleswith diameters of about 20 to 100 nanometers and a large surface area byreaction between diethyl zinc and hydrogen sulfide comprising thefollowing steps:maintaining a solution of toluene and H₂ S at atemperature within the range from about 100° C. to about -40° C.;saturating said solution of toluene with H₂ S; adding a solution ofdiethyl zinc and toluene to said H₂ S saturated toluene solution at arate within the range from about 0.01 moles/hour to about 1 mole/hourfor a reaction on the 0.1 mole scale; maintaining H₂ S saturation ofsaid reaction mixture by adding H₂ S at a greater rate than diethylzinc; recovering a pure, finely divided powder; and drying said powderby heating while under a vacuum of less than 0.01 torr.
 10. The methodof claim 9 wherein said solution of an organic solvent and H₂ S ismaintained at a temperature within the range from about -40° C. to about-100° C. and at a H₂ S concentration of about 1 mole per liter.